I've added a line "HTTPSClient.UseIPv6 = true;" directly before the "HTTPSClient.Get(URL);", but I'm now getting the exception of a connection error, code 96260 and I see no request going out over Wireshark. The address I'm making the request to shows in the URL parameter as:

HTTP://[2a55:1455:e055:1:90a3:3124:2ffb:9f65]:80/sub

I see this error code mentioned elsewhere on these forums occurring with invalid characters being passed, but don't understand what could be wrong with this character set...?